The present invention relates to a process for the preparation of catalytic systems for the oxidative dehydrogenation of alkylaromatics, in particular ethylbenzene, to the corresponding alkenylaromatics, in particular styrene, or of paraffins to the corresponding olefins.
Styrene, which is an important intermediate for the production of plastic materials, is mainly used in the production of polystyrenes (crystal GPPS, high impact HIPS and expandable EPS), acrylonitrile-styrene-butadiene (ABS) and styrene-acrylonitrile (SAN) copolymers, and styrene-butadiene rubbers (SBR).
Styrene is currently produced mainly by means of two processes: by the dehydrogenation of ethylbenzene (EB) and, as co-product, in the epoxidation of propylene with ethylbenzene hydroperoxide with catalysts based on molybdenum complexes.
An alternative method for the production of the monomer is the dehydrogenation of ethylbenzene with the contemporaneous oxidation of hydrogen which can be carried out in the presence or in the absence of oxygen.
Oxidative dehydrogenation in the absence of oxygen consists in the use of one or more metallic oxides which, in addition to catalyzing the dehydrogenation reaction of ethylbenzene, is capable of oxidizing the hydrogen produced, by means of the oxygen available in the oxide itself, in order to favour the equilibrium shift towards the formation of styrene (STY) by means of the following reaction 
It can be seen from reaction (1) that the catalyst also participates in the reaction stoichiometry, acting as reagent: at the beginning of the reaction, it is in an oxidized state (catox) capable of releasing part of its oxygen and becoming transformed into a reduced species (catred). In order to make the reaction catalytic, the reduced catalyst must be able to easily re-acquire oxygen to be transformed into the starting oxidized species, which can be used for a new oxidative dehydrogenation cycle, by means of the following reaction: 
This particular way of carrying out dehydrogenation offers the same advantages as traditional oxidative dehydrogenation, i.e. in the presence of oxygen, allowing the necessary heat supply for the dehydrogenation and the equilibrium shift of the dehydrogenation reaction towards the products.
The idea of effecting the oxidative dehydrogenation of hydrocarbons in the absence of an oxidizing gas was already known and described in the first half of the sixties"" by U.S. Pat. No. 3,118,007 of Bayer. This patent claims a process for the dehydrogenation of hydrocarbons in the absence of oxidizing gases and with catalysts based on iron oxides which also act as oxygen carriers. The possibility of operating under fluid bed conditions in order to be able to continuously remove the catalyst to be subjected to a re-oxidation phase and then recycled to the reaction phase, is also described.
Various patents have been filed in the last few years again relating to oxidative dehydrogenation without oxidizing gases of which the most significant are the following.
EP-482276 of FINA describes a process whereby a total conversion of ethylbenzene is obtained already at 505xc2x0 C. With a catalyst, acting as oxygen carrier, which, once exhausted, can be regenerated in a second reactor by treatment with air. The catalyst, containing transition metal oxides, preferably based on vanadium supported on magnesium, has a high dehydrogenating activity as well as a strong tendency to release structural oxygen by the combustion of hydrogen. The results indicated in this patent show that combustion is the most critical phase of the reaction: at the beginning of the catalytic activity, in fact, styrene is produced with a low selectivity together with a high quantity of carbon oxides deriving from the combustion of ethylbenzene and/or styrene. In the same patent, it is shown that a partial pre-reduction of the catalyst, by treatment with carbon monoxide, allows its high oxidizing capacity to be moderated, obtaining high selectivities to styrene already in the first phases of activity. In this case however the conversion drops rapidly and becomes stable in a short time at values around 50%.
GB-2297043 of BASF claims the use of a catalyst consisting of a mixed oxide based on bismuth, titanium, lanthanum, potassium and treated with a noble metal, for the oxidative dehydrogenation of ethylbenzene in the absence of oxygen. The results indicated do not allow the catalytic performances over a period of time to be accurately evaluated. The patent text discloses that the catalyst is initially extremely active but has a low selectivity with the formation of compounds deriving from the combustion of hydrocarbons. As already observed in the case of the FINA patent, as the reaction proceeds, the catalyst becomes less active and more and more selective until it reaches a maximum value.
The same applicants have recently filed a patent application (IT-MI99A001242) which describes a catalytic system consisting of:
a vanadium oxide;
a bismuth oxide;
and a carrier based on magnesium,
wherein the vanadium, expressed as V2O5, is in a quantity ranging from 1 to 15% by weight, preferably from 2 to 10%, the bismuth, expressed as Bi2O3, ranges from 2 to 30% by weight, preferably from 5 to 25% by weight,
the complement to 100 being the carrier.
The magnesium-based carrier is preferably selected from:
carriers consisting of magnesium oxide;
carriers consisting of magnesium oxide and zirconium oxide;
carriers consisting of magnesium and aluminum hydrotalcites.
The process for preparing the catalytic system described in the above Italian patent application can be essentially carried out by means of the following steps:
preparation of the solutions or suspensions based on derivatives of the components of the catalytic system;
mixing of the solutions or suspensions prepared until gelation of the mixture;
drying of the gel obtained;
calcination of the dried solid at a temperature ranging from 550 to 780xc2x0 C.
We have now found a different preparation process for obtaining the catalytic system described above, which can be used when the carrier consists of magnesium oxide.
The catalytic system thus obtained, with respect to the known catalysts described above, not only provides better selectivity characteristics, above all at the beginning of the reaction, and obtains a higher total productivity, but also has a longer life duration.
The process, object of the present invention, for preparing catalytic systems consisting of:
a vanadium oxide;
a bismuth oxide;
and a carrier consisting of magnesium oxide,
wherein the vanadium, expressed as V2O5, is in a quantity ranging from 2 to 35% by weight,
the bismuth expressed as Bi2O3, ranges from 2 to 40% by weight,
the complement to 100 being the carrier, is characterized in that it essentially comprises the following steps:
preparation of solutions based on derivatives of the components of the catalytic system;
mixing of the solutions prepared and optional aging;
drying of the solution obtained;
first heating, in the presence of air, of the solid obtained from the drying at a temperature ranging from room value to a temperature of between 290 and 310xc2x0 C., for a time ranging from 1 to 3 hours;
optional second heating, in the presence of air, of the solid for a time ranging from 0.5 to 2 hours at a constant temperature, ranging from 290 to 310xc2x0 C., reached in the first heating;
additional heating, in the presence of air, of the solid for a time ranging from 2 to 4 hours at the calcination temperature ranging from 600 to 850xc2x0 C.;
calcination, in the presence of air, of the solid at a constant temperature, ranging from 600 to 850xc2x0 C., reached in the additional heating, for a time ranging from 8 to 16 hours.
The catalytic system obtained in accordance with the invention, in its calcined form at a temperature ranging from 600xc2x0 C. to 850xc2x0 C., has an X-ray diffraction spectrum, registered by means of a vertical goniometer equipped with an electronic diffracted radiation count system and using copper Kxcex1 radiation (wave length=1,5418 xc3x85), of the type indicated in FIGS. 1-6, containing reflections whose inter-planar distance values are indicated in Table 2.
The catalytic system is characterized by the presence of at least two of the following three phases:
the ever present MgO crystalline phase;
the new Bi/Mg/V/O crystalline phase (with an unknown structure),
the BiMg2VO6 crystalline phase.
The relative quantity of Bi/Mg/V/O and BiMg2VO6 crystalline phases is determined not only by the chemical composition, but also by the temperature and calcination procedure.
In particular:
on effecting the calcination at a temperature ranging from 600 to 800xc2x0 C., a catalytic system is obtained, characterized by a Bi/Mg/V/O crystalline phase, with an X-ray diffraction spectrum as indicated in Table 1, ranging from 10 to 45%, an MgO crystalline phase and a BiMg2VO6 crystalline phase, which is sometimes not present.
on effecting the calcination at a temperature higher than 800xc2x0 C. and lower than or equal to 850xc2x0 C., a catalytic system is obtained, characterized by an MgO crystalline phase and a BiMg2VO6 crystalline phase.
In the case of a calcination carried out at 600 to 800xc2x0 C., it can be observed that the catalytic system is characterized by the presence (FIGS. 1-5, Table 2) of an MgO crystalline phase, a new Bi/Mg/V/O crystalline phase (with an unknown structure), with an X-ray diffraction spectrum as indicated in Table 1, and the BiMg2VO6 crystalline phase which is sometimes not present (FIG. 1). Other crystalline phases such as (for example): Mg3V2O8, Mg(Mg,V)O4, etc. may also be occasionally present, in smaller quantities.
In the case of a calcination carried out at a temperature higher than 800xc2x0 C. and lower than or equal to 850xc2x0 C., it can be observed that the catalytic system is characterized by an X-ray diffraction spectrum as illustrated in FIG. 6.
In the catalytic system claimed, the vanadium, expressed as V2O5, is preferably in a quantity ranging from 5 to 30% by weight, more preferably from 10 to 25%, and the bismuth, expressed as Bi2O3, preferably ranges from 10 to 20% by weight.
The catalytic system according to the invention can be applied to any dehydrogenation technology of ethylbenzene whether it be fixed, fluid or mobile bed.
The process, further object of the present invention, for the oxidative dehydrogenation of alkylaromatics, in particular ethylbenzene, into the corresponding alkenylaromatics, in particular styrene, or paraffins into the corresponding olefins, substantially consists in reacting the alkylaromatic or paraffin, optionally in the presence of a diluent, in a reactor, operating at a temperature ranging from 350 to 750xc2x0 C., preferably from 370 to 550xc2x0 C., at a pressure ranging from 0.1 to 30 psia and with a GHSV space velocity ranging from 0.01 to 10 secxe2x88x921, preferably from 0.1 to 1 secxe2x88x921 (normal-liters of hydrocarbon/secxc3x97liter of catalyst), with the catalytic system described above and regenerating said catalytic system in a regenerator by burning the coke deposited during the reaction phase, operating at a temperature higher than 400xc2x0 C.
The oxidizing medium used in said process can be oxygen and/or air.
The optional diluent can be, for example, N2, CH4, H2Ovapour, CO, CO2, etc.
In order to avoid having substantially aldehydes and/or ketones in the products obtained, it is advisable to use charges without oxygenated compounds (in particular peroxides).
Some examples are provided for a better illustration of the present invention without limiting its scope in any way.